Conventionally, there has been disclosed a method of manufacturing an electrode of a solar cell, comprising: forming a lower electrode layer by coating and baking a first solution coated on a photoelectric conversion semiconductor layer; and forming an upper electrode layer by coating and baking a second solution on a photoelectric conversion semiconductor layer. In this method, the first solution includes ultrafine metal particles having a grain size of 0.03 μm or less which are dispersed in an organic solvent having a low molecular number of about 100 to 200, and the second solution includes ultrafine metal particles in the same or higher weight concentration than in the first solution used in the above-described formation of the lower electrode layer (For example, see Claim 1, paragraph [0024], and paragraph [0035] of Patent Reference 1: Japanese Patent No. 3287754). In this method of manufacturing metal electrodes, ultrafine metal particles are dispersed in the solution, and the viscosity of the solution is controlled to be about 10000 cps. The solution is coated on the photoelectric conversion semiconductor layer by a screen printing method or the like. After that, the metal electrode (the upper electrode layer or the lower electrode layer) is formed by baking the solution for 30 minutes or more at 100 to 250° C., and preferably at 250° C.
In the thus constituted method of manufacturing metal electrodes of a solar cell, the solution dispersing ultrafine metal particles in the organic solvent is coated on the photoelectric conversion semiconductor layer, and subsequently the solution is baked at a low temperature of 100 to 250° C. By this method, it is possible to obtain metal electrodes having high reflectance, high conductivity, and large area without using high-vacuum process.
An electric-conductive composition consisting of Ag powder; at least one metal selected from V, Mo, and W; glass frit; and an organic vehicle is disclosed as a composition used in a method of manufacturing electrodes of a solar cell (for example, see claim 2, paragraph [0022] and paragraph [0031] of Patent Reference 2: Japanese Unexamined Patent Application, First Publication, No. Hei 10-326522). In the above-described Patent Reference 2, a base material printed with the electric-conductive composition for a solar cell is subjected to baking for five minutes at 550° C., thereby forming a Ag electrode. By using the electric-conductive composition for a solar cell described in Patent Reference 2, it is possible to remarkably enhance the sinterability of an Ag electrode. Specifically, it is possible to improve the electric conductivity and film-strength of an electrode under a low-temperature baking at 700° C. or less. Therefore, it is possible to contribute to cost-reduction by decreasing the baking temperature, and formation of electrodes in a case where a treatment temperature of a substrate element is restricted by an upper limit.
Patent Reference 3 discloses an electric-conductive paste formed by mixing a metal powder, an oxide powder, and a vehicle. The metal powder is a powder of at least one metal selected from a group consisting of Ag, Cu, and Ni. The oxide powder is a crystalline powder of a complex oxide that includes at least one selected from a group consisting of Bi, Fe, and Ag, and at least one selected from group V elements and group VI elements of a periodic table (for example, see Claims 1 to 3, paragraph [0009], [0031] of Patent Reference 3: Japanese Unexamined Patent Application, First Publication, No. H11-329070). In the above-described Patent Reference 3, an electrode is formed by baking a wafer printed with the electric-conductive paste, where the maximum baking temperature is 750° C. By using the electric-conductive paste described in the Patent Reference 3, it is possible to form an electrode having low contact resistivity and high bonding-strength certainly.
Further, Patent Reference 4 discloses a method of manufacturing a solar cell element comprising: forming an area of one type electric-conductivity on one main surface of a semiconductor substrate that provides another type conductivity; forming an anti-reflection film on the main surface of the semiconductor substrate; and bake-printing a raw material of an electrode on another main surface of the semiconductor substrate opposite to the anti-reflection film, wherein the raw material comprises silver powder, an organic vehicle and glass frit, and the raw material further comprises one or a plurality of species selected from Ti, Bi, Co, Zn, Zr, Fe, and Cr components (for example, see Claim 1, paragraph [0027], paragraph [0039] of Patent Reference 4: Japanese Unexamined Patent Application, First Publication, No. 2001-313400). In the above-described Patent Reference 4, a solar cell element is formed by bake-printing the paste at 700° C. In the method shown in the Patent Reference 4, even when the raw material of an electrode is coated and baked on the anti-reflection film, it is possible to obtain a solar cell element having a satisfactory ohomic contact (fill factor), and a strong tensile-strength.
A method of forming an electric-conductive coating by coating and heating an electric-conductive composition has been conventionally disclosed as a method of forming a metal electrode on a semiconductor substrate using a raw material containing fine metal particles, for example, an electric-conductive paste. In this conventional method, the electric-conductive composition contains particulate silver compounds, reducing agent and a binder, where the silver compounds may be silver oxide, silver carbonate, and silver acetate or the like (for example, see claims 1 to 3, 11, lines 32-33 on page 3 of Patent Reference 5: PCT International Patent Application, Publication No. 2003/085052). According to the method of Patent Reference 5, without using a high-temperature film formation conditions, it is possible to obtain an electric-conductive film of low volumetric resistivity and high conductivity similar to those of metallic silver.
There has been disclosed an electric-conductive paste and a method of manufacturing a solar cell using the electric-conductive paste. The electric-conductive paste comprises an organic binder, a solvent, glass frit, and an electric-conductive powder, wherein the paste contains a powder of metal or metal compound including at least one selected from Ti, Bi, Zn, Y, In, and Mo, and having an average grain size of not less than 0.001 μm and less than 0.1 μm. The solar cell is produced by printing or coating the electric-conductive paste on an anti-reflection layer of a silicon semiconductor substrate, and subsequently baking the substrate. (For example, see claims 1, 6, paragraphs [0021] of Patent Reference 6: Japanese Unexamined Patent application, First Publication No. 2005-243500). In the electric-conductive paste shown in the above-described Patent reference 6, it is preferred to bake the substrate printed or coated with the electric-conductive paste at a temperature of 550 to 850° C., thereby forming the electrode. According to the Patent Reference 6, additive of ultra fine particles are uniformly dispersed. By baking the substrate, it is possible to form a surface electrode having high conductivity and superior bonding strength between the semiconductor and the electric-conductive paste interposing the anti-reflection layer.